Rietveld and pair distribution function study of Hagg carbide using synchrotron X-ray diffraction

• Fischer-Tropsch (FT) synthesis is an important process in the manufacturing of hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The reduced iron catalyst reacts with carbon monoxide and hydrogen to form bulk Fe5C2 Hagg carbide (chi-HC) during FT synthesis. Arguably, chi-HC is the predominant catalyst phase present in the working iron catalyst. Deactivation of the working catalyst can be due to oxidation of chi-HC to iron oxide, a step-wise decarburization to cementite (theta-Fe3C), carbon formation or sintering with accompanying loss of catalytic performance. It is therefore critical to determine the precise crystal structure of chi-HC for the understanding of the synthesis process and for comparison with the first-principles ab initio modelling. Here the results of high-resolution synchrotron X-ray powder diffraction data are reported. The atomic arrangement of chi-HC was confirmed by Rietveld refinement and subsequent real-space modelling of the pair distribution function (PDF) obtained from direct Fourier transformation. The Rietveld and PDF results of chi-HC correspond well with that of a pseudo-monoclinic phase of space group Pi [a = 11.5661 (6) A, b = 4.5709 (1) A, c = 5.0611 (2) A, alpha = 89.990 (5)degrees, beta = 97.753 (4)degrees, gamma = 90.195 (4)degrees], where the Fe atoms are located in three distorted prismatic trigonal and one octahedral arrangement around the central C atoms. The Fe atoms are distorted from the prismatic trigonal arrangement in the monoclinic structure by the change in C atom location in the structure.

Ämnesord

NATURVETENSKAP  -- Fysik -- Acceleratorfysik och instrumentering (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Accelerator Physics and Instrumentation (hsv//eng)

Nyckelord

Hagg carbide

Fischer-Tropsch synthesis

crystal structure

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